Introduction Susceptibility of healthcare workers (HCW) to influenza is relevant in terms of sickness absence, productivity and onward transmission of infection from carer to patient, a particular issue with a novel influenza strain. The aim of this study was to determine the seroprevalence of antibodies against the virus in unvaccinated healthcare workers in Lothian, south east Scotland during October- November 2009 (after the first wave of infection, but before the expected increase in cases over the winter period).

Methods Unvaccinated employees (n=493) were recruited within days of the start date of the HCW vaccination programme and prior to most being vaccinated. Participants had a serum sample taken and completed a short questionnaire recording sex, age, occupation, and self-reported history of flu-like symptoms or illness since the start of the pandemic period. Serology specimens were analysed in the West of Scotland Specialist Virology Centre in Glasgow using microneutralisation assays at a dilution of 1:40.

Results The prevalence of seropositivity in HCWs mid-pandemic was 10.3% (95% CI 7.7 to 13.0%), which was higher than pre-pandemic HCW seropositivity rate by 3.7% (95% CI of increase 0.3% to 7.3%, p=0.048). Seropositivity rates for frontline and non-frontline HCWs were similar.

Conclusion To our knowledge, this is the first study in the United Kingdom to quantify the level of seropositivity to influenza A(H1N1) in unvaccinated HCWs pre- and mid-pandemic. Only 10.3% of HCWs were seropositive for influenza A(H1N1) mid-pandemic, so the great majority were still susceptible to infection at the introduction of the vaccination programme.